Help Understanding AIS

[CaptTom]

Originally Posted by Paul L

Not sure why this would be a good norm. How about when anchored in fog or poor visibility, or any emergency when the engines are off and you are in a channel?

I have an three-position switch: on-off-auto. Auto means only when an engine is running. Honestly, I don't use "auto" anywhere near as much as I thought I would, but it's handy and I really should use it more. I'd hook it up this same way if I had it to over again.

[Montanan]

Originally Posted by CaptTom

I have an three-position switch: on-off-auto. Auto means only when an engine is running. Honestly, I don't use "auto" anywhere near as much as I thought I would, but it's handy and I really should use it more. I'd hook it up this same way if I had it to over again.

[Paul L]

Originally Posted by CaptTom

I have an three-position switch: on-off-auto. Auto means only when an engine is running. Honestly, I don't use "auto" anywhere near as much as I thought I would, but it's handy and I really should use it more. I'd hook it up this same way if I had it to over again.

CF has some interesting posts at times. So the thing you promoted as good and I questioned as not good in many cases, you now post that you don't actually use.
This extra 3 way switch on an AIS seems like unnecessary complication and of dubious value.

[Pete7]

Originally Posted by Wayne hoath

People leave AIS on so they can look on marine traffic and see their boats. No. I don’t get it either.

We did a quick bit of naming and shaming on the club FB page. That solved the problem. Sadly Premier, Haslar and Clarence Marinas are full of them

[Dockhead]

Originally Posted by Captain Puget

AIS is a great tool, especially in high traffic areas and shipping lanes. However, I am not understanding why I am observing certain phenomenon. The Coast Guard Rules state that many vessels (mostly commercial vessels) must have AIS broadcasting for a minimum of 15 minutes before they are underway. So why am I seeing the following?

Observations:
1) In a marina/ moorage area, there can be many vessels that just leave their AIS on even while tied up or moored (often for months at a time!). This leads to many false AIS warnings that cause me to turn off my receiver near moorages so that my chart plotter is not bombarded with so many alerts that it can become virtually unusable. Is this just sloppiness on the vessels' captains part or ???

It's sloppiness on your part, not the vessels' captains -- you need to read the manual and understand how the alarms work. You do not need to "turn off the receiver" in order to stop alarms from moored vessels. AIS alarms should be configured to suit the waters you are sailing in -- one size does not fit all.



AIS alarms are useless and actually are a dangerous distraction inside harbors and approaches where vessels are following channels and are not on steady courses. The alarms should be OFF in such circumstances.


The contrary case is in open water away from traffic lanes, where you will want to adjust the alarms to a greater CPA and TCPA to help you be aware of traffic early. Offshore I often use 30 minutes and 2 miles, or even 1 hour.

Originally Posted by Captain Puget

. . . 2) Some vessels seem to turn off AIS or stop broadcasting while stationary. The best example are the State Ferries here in Washington. While boarding vehicles, the ferries stop broadcasting AIS, and only start rebroadcasting just as they cast off and leave the dock (while blasting their horns and yelling for all small craft to get out of their way over the VHF). They definitely don't adhere to the 15 minute rule. You can't tell if thy are about to leave or moored at the terminal for an extended period. I spoke with a ferry captain, and he insisted they do not turn AIS off while boarding (contrary to many observations by me).

I would appreciate any enlightenment, please.

You are not correctly informed about the 15 minute rule -- it is AT LEAST 15 minutes not AT MOST. There is no rule against broadcasting AIS when docked and some ports require ships to continuously use their AIS.


I know that vessels broadcasting AIS when they are berthed irritates some people, but in my opinion this is purely a problem of using alarms incorrectly. I will take the opposite position and say that I wish that people would leave their AIS on as much as possible -- I find it useful to know how many vessels are where and what types, and it can be really useful to see which berths are occupied in a harbor before you get close enough to see.


Mine is on whenever I'm on board -- I keep the instruments on as I like to look at the wind and weather and watch other traffic on AIS, and I like for my friends to be able to see where I am.



Now you do occasionally see vessels on the hard broadcasting AIS -- I would agree that that is a bit much!

[Dockhead]

Originally Posted by CaptTom

I have an three-position switch: on-off-auto. Auto means only when an engine is running. Honestly, I don't use "auto" anywhere near as much as I thought I would, but it's handy and I really should use it more. I'd hook it up this same way if I had it to over again.

That looks like a good setup.


Note that if you wish to be able to receive AIS but not broadcast it, most AIS sets have contacts for a "silent mode" switch, so that you can stop broadcasting without shutting down the set.

[capn_billl]

Originally Posted by Paul L

On your #1 issue:
If you don't want 'false' alarms then change the alarm criteria on your receiver. Some units, like my Vesper, have alarm profiles that you can setup. For example one for offshore and one for harbor. The criteria to alarm is very different between the two. Other systems offer the possibility to only alarm on vessels moving faster than a set criteria. Alarms are the receivers responsibility.
There is no requirement to turn off your AIS when moored.

On #2.
As noted above the transmission rate is based on vessel speed. The AIS system is not a guranteed delivery system. The packet is broadcast, your system may or maynot receive it. If you drop one or two packets then the wait is more than doubled to see the next update.

Here is the issue with that. The AIs uses the vehicle GPS, or internal GPS to determine speed.

This results in random movements of several feet, and several miles per hour when tied loosely.

When passing by, I see an arrow pointing on a collision course for several seconds, followed by it pointing away as the boat rocks back into position. You also get random movements from GPS fluctuations. It requires zooming in to determine if the boat has actually left the slip, or just random movement. TIMES 50, for EACH boat in the marina while I'm busy trying to locate MY slip, and enter.

[Paul L]

Originally Posted by capn_billl

Here is the issue with that. The AIs uses the vehicle GPS, or internal GPS to determine speed.

This results in random movements of several feet, and several miles per hour when tied loosely.

When passing by, I see an arrow pointing on a collision course for several seconds, followed by it pointing away as the boat rocks back into position. You also get random movements from GPS fluctuations. It requires zooming in to determine if the boat has actually left the slip, or just random movement. TIMES 50, for EACH boat in the marina while I'm busy trying to locate MY slip, and enter.

Since the broadcast position, sog and cog from the AIS targets is at a very low rate, once every 30 seconds or greater, it can't be jumping around. Your own boats sog and cog calculated by the gps can appear to bounce around because it is updated at 1 per second.
Set your alarm limits correctly and you should have minimum false alarms.
If you have AIS alarms enabled and drive down a fairway with ais targets on each side, of course you will get alarms based on cpa or tcpa. Don't want them, then disable ais alarms on your vessel.

If you drove down the same fairway with your radar on and an alarm band enabled you would also get alarms from the boats on either side.

[capn_billl]

If you want to use your AIS as a burglar alarm, (not its intended purpose), wire it to the engine.

A thief is likely to start the engine at some point, even if stealing a sailboat.